1. Field of the Invention
The present invention relates to a liquid crystal polyester resin composition.
2. Description of Related Art
Because of their excellent thermal resistance and mechanical physical properties as well as their excellent flowability in molten state, i.e., processability, melt liquid crystallinity polyester resins that exhibit liquid crystallinity in molten state (hereinafter, such resin is referred to as liquid crystal polyester resin) have been used in various fields as molding materials which can be subjected to precise molding. In recent years, in the electric and electronic fields where liquid crystal polyester resins are used very often, downsizing, lightening, high-speed processing and energy saving are more and more required for products represented by cellular phone parts and optical disc pickup parts. Following this, more light-weight (low specific gravity) resins have been required as materials having necessary mechanical physical properties and processability.
Since an injection molded article of a liquid crystal polyester resin alone has strong anisotropy, the molding is difficult, and usually it is used as a composite material added with inorganic fillers, such as glass fiber. As for a glass fiber containing liquid crystal polyester resin composition, for example, JP-B 1-27103 discloses that glass fibers having a specific fiber diameter and fiber length are excellent in respect of the balance between moldability and physical properties. JP-B 7-68409 discloses the mixing two kinds of glass fibers having different mean fiber lengths, in a specific ratio. JP-A 4-292651 discloses glass fiber having specific weight average fiber length and specific number average fiber length. Moreover, JP-A 63-101448 discloses compounding glass fibers having ten or more of the aspect ratio, and 0.15 to 0.6 mm of the fiber length.
JP-A 6-240114 discloses glass fibers having a specific fiber length distribution.
It is more advantageous generally to add much of such glass fibers, in order to reduce the anisotropy, and commercial liquid crystal polyester resin compositions for injection molding contain 20 to 50 wt %. However, as for that containing much amount of glass fibers, the specific gravity as a liquid crystal polyester resin composition becomes large, it has already become difficult to use it, in the field where a light weight property is required,
An object of the present invention is to solve the above-mentioned problem, and to provide: a liquid crystal polyester resin composition which is low in specific gravity, and excellent in processability, mechanical properties, and heat resistance; a process for producing the same; and a molded article using thereof.
As a result of extensive studies, the present inventors have found that a specific liquid crystal polyester resin having low specific gravity, and excellent processability and mechanical properties can be obtained by compounding a specific amount of glass fibers having a specific number average fiber diameter and a specific number average fiber length, thus completed the present invention.
Namely, the present invention relates to [1] a liquid crystal polyester resin composition, which comprises 5-20 parts by weight of glass fiber having a number average fiber diameter after molding of 2-20 xcexcm, and a number average fiber length after molding of 210-500 xcexcm; and 100 parts by weight of a liquid crystal polyester resin containing the following structural units (I), (II), and (III), or the following structural units (I), (II), (III) and (IV); and the sum of (I), (II), (III) and (IV) is 95% by mole or more, and the flexural modulus thereof measured using a test piece of 0.5 mm thickness is 25 GPa or more. 
The present invention also relates to [2] a molded article obtained by using the liquid crystal polyester resin composition of the above [1].
Further, the present invention relates to [3] a process for producing a polyester resin composition of [1], wherein a melt-kneading extruder equipped with a screw is used and the extruder has an upper stream side supplying portion at the upper stream part of the extrusion direction, and a lower stream side supplying portion at the lower stream part from the upper stream side supplying portion, and the ratio (L/D) of the distance (L) between the upper stream side supplying portion and the lower stream side supplying portion to the diameter (D) of a screw is 4-30 (L and D are the same scale units); and under screw rotation, 90% or more of the total supplying amount of the liquid crystal polyester resin and less than 5% of the total supplying amount of the glass fiber are supplied from the upper stream side supplying portion, and less than 10% of the total supplying amount of the liquid crystal polyester resin and 95% or more of the total supplying amount of the glass fiber are supplied from the lower stream side supplying portion.